1. Field of Invention
The invention relates to a shift control apparatus for an automatic transmission and, more particularly, to an art for quickening a shift requiring release and engagement of two each of different engagement elements between gear speeds, i.e., simultaneous changeover of two elements.
2. Description of Related Art
An automatic transmission, as well known, achieves a plurality of gear speeds due to the change of a gear ratio by switching a power transmission path through shift elements constituted by a planetary gear, by means of engagement and release of frictional engagement elements. In an effort to perform engagement and release of the engagement elements during the shift with an as-simple-as possible hydraulic control while preventing the generation of transmission shock, generally, an operation of the engagement elements for a shift-up/down is based on additionally engaging another engagement element or releasing one of the engagement elements that are engaged with respect to a plurality of or single engagement element(s) in an engaged state, in order to achieve a predetermined gear speed. A so called changeover of the engagement elements comprises releasing one engagement element that is engaged while engaging another engagement element, in a case in which it is absolutely necessary because of the structure of the gear train.
Meanwhile, there has been a trend for automatic transmissions to be multi-speed so as to match the improved drivability and because of demands for improved energy conservation by reducing fuel consumption. The multi-speed automatic transmission is generally realized in an embodiment where an acceleration or deceleration speed, assisted by an overdrive or underdrive gear, is added to a transmission mechanism constituted by a planetary gear set for multiple speeds. However, another embodiment, as disclosed in Japanese Patent Application Laid Open No. 4-219553, achieves multiple speeds by setting two systems, a high and a low, of input to a Ravigneaux type planetary gear set.
The aforementioned gear train with multi-speed offers a wider choice of options for the gear speed matched to a running condition of a vehicle. Therefore, a changeover operation of the engagement elements requires not only a simple changeover of two elements, but also a complicated changeover of four elements. An example that requires the changeover of the four elements is a so called jump shift in which the shifting is suddenly made to a predetermined gear speed among a plurality of gear speeds. In any case, where this kind of multiple changeover of the four elements is performed, how to control the order and the timing of the engagement and release of each engagement is an important issue. If the control is not appropriate, smooth progress of the shift in the transmission mechanism is hindered and continuity in the shifting is lost. Accordingly, problems are caused, such as a step-like shock is generated during the shift, shock at the completion of the transmission becomes very large, or a longer time than necessary is required for the shift.
Therefore, it is an object of the invention to provide a shift control apparatus for an automatic transmission that prevents the period of shift from extending by quickening the progress of the shift at the time of shift that requires engagement and release of the four engagement elements. It is a further object of the invention to quicken the progress of the shift at the latter half after passing the intermediate speed when the shifting shifts through the intermediate speed.